1. Field of the Invention
The present invention relates to the treatment of gas mixtures, in particular the retention of noble gases in the exhaled air of ventilated patients. Especially, the present invention relates to the use of a selective gas separation membrane for retaining noble gases in the respiration gas of ventilated patients.
2. Description of the Invention
Among noble gases only xenon shows an anaesthetic effect under conditions of standard atmospheric pressure. This effect was demonstrated in 1941 by the Russian scientist Nikolay Vasilievich Lazarev.
The narcotic effect of xenon is 1.5 times stronger than that of nitrous oxide. Due to its extreme low blood solubility xenon will be exhaled more quickly than all other anaesthetics hitherto known. Additionally, xenon is environmentally friendly since it is neither harmful to the ozone layer nor is it a green-house gas. Xenon is inflammable and harmless to pregnant women. Apart from its anaesthetic property xenon is beneficial for the protection of the brain function of the patient. Xenon is particularly suitable for patients suffering from cardiovascular problems because during the anaesthesia with xenon the circulatory conditions of the patient remain extremely stable.
Due to its properties xenon shows advantages for specific indications compared to other anaesthetics. The material costs admittedly will be higher. The total costs for the treatment, however, will be markedly lower due to the advantageous activity profile, the minor side effects and the protective properties for organs while using xenon.
For the application in medicine the production of xenon will markedly rise in the future. But due to its chemical properties and its low availability and the costs associated with its production xenon is no alternative for nitrous oxide or established anaesthetics but is to complement them.
One possibility of cost-cutting will be to recycle and reuse the used xenon.
There are some cryogenic processes in the prior art.
DE 44 11 533 C1 describes an anesthesia apparatus having a recovery installation for xenon. In the recovery installation the pre-purified exhaled air is compressed and led into a pressure vessel which is included in a cooling device.
The pressure vessel is cooled by means of the cooling device so that the recovered xenon will be liquefied. The xenon from the exhaled air will be collected in the pressure vessel in a liquid state. From there, the xenon will be led back to the patient.
WO 98/18718 describes an apparatus and a process for purifying and recovering xenon in the anesthetic system, whereas xenon is collected in the liquid state in a cryogenic vessel after its purification and lead back to the patient.
DE 196 35 002 A1 describes a process for the online-recovery of xenon from narcotic gas, whereas the exhaled air is contacted with a cooling surface, the temperature of which is below the melting point of the component to be recovered. Hereby xenon will be separated by freezing and the impurities will be withdrawn in vacuo over the top gas.
WO 01/24858 A describes a system and a process with which gases, in particular humid gases such as expiration gases or exhaust gas from anesthetic instruments can be collected for recycling. The gas will be converted into a compressed form such as cold-worked or compressed gas in the gas compression vessel.
The mentioned systems are part of an anaesthetic system and are intended to directly recirculate the xenon to the patient during anaesthesia. Said systems and processes are accompanied by several problems regarding the instruments and the costs.
Hence, recently alternatives were searched for which allow a more simple and thus less expensive recycling of the xenon.
In this connection gas separation by an appropriate semi-permeable membrane, a so-called selective gas separation membrane plays an important role.
The separation of liquid, gaseous and vaporous mixtures of fluids by membranes is known in various processes. At least one of the components of the applied fluids is retained by the membrane and discharged in the form of a so-called retentate. At least another component of the fluid mixture will be able to permeate the membrane, which then will be discharged as permeate on the other side of the membrane.
Recently, however, techniques have been developed with which it was possible to produce sufficiently thin and therefore sufficiently permeable films for gas separation which are free of voids and mechanically stable. These types of membranes are based on very thin, nonporous and gas selective films on porous supporting layers.
From the prior art for example EP 428 052 gas separation membrane is known which is a semi-permeable composite membrane.
DE 697 17 215 T2 discloses a process for gas recovery, in particular of noble gases, from plasma display panel sealing furnaces by membrane separation.
DE 103 00 141 A1 describes an oxygen enrichment method from air by simultaneously reducing the carbon dioxide concentration in a closed or partially closed unit of space by means of a gas-purification membrane system. The used membranes have active layers of for example polysulfone, polyoctylmethylsiloxane, polyetherimide, silicon, ethylcellulose, polyphenylene oxid, polysulfone, polycarbonate as well as combinations thereof.
From EP 1 086 973 A2 gas separation means of polyimide, such as films, coatings and membranes are known, which are adapted for numerous fluid separation applications.
Use of gas separation membranes for the retention of noble gases in the exhaled air of ventilated patients has not been described so far.